Systems and methods for enhancing mobility of atomic or molecular species on a substrate at reduced bulk temperature using acoustic waves, and structures formed using same

ABSTRACT

Under one aspect of the present invention, a method for enhancing mobility of an atomic or molecular species on a substrate may include exposing a first region of a substrate to an atomic or molecular species that forms a molecular bond with the substrate in the first region; directing a laser pulse to a second region of the substrate so as to generate an acoustic wave in the second region, the acoustic wave having spatial and temporal characteristics selected to alter the molecular bond; and transmitting the acoustic wave from the second region to the first region, the acoustic wave altering the molecular bond between the substrate and the atomic or molecular species to enhance mobility of the atomic or molecular species on the substrate in the first region.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.FA8802-09-C-0001 awarded by the Department of the Air Force. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This application relates to the mobility of molecular species on asubstrate.

BACKGROUND OF THE INVENTION

A variety of gas-phase techniques for depositing materials on asubstrate are known in the art, including physical vapor deposition(PVD) based techniques such as evaporation, sputtering, molecular beamepitaxy (MBE), and pulsed laser deposition (PLD), and a wide variety ofchemical vapor deposition (CVD) based techniques, including atomic layerdeposition (ALD). These techniques are ubiquitous in the development ofnovel materials and films that have a wide range of applications fromconsumer electronics to biology or medicine, from architecturalengineering to aerospace. Both PVD and CVD based techniques may includeexposing the substrate to an atomic or molecular species at a set ofprocessing parameters that are selected based on the depositiontechnique to be used, the material to be deposited, and the substrateupon which the material is to be deposited.

For example, FIGS. 1A-1B schematically illustrate structures that may beformed during a previously known gas-phase material depositiontechnique. As illustrated in FIG. 1A, substrate 110 may be exposed to anatomic or molecular species 120 that is in the gas phase. In theevaporation and MBE techniques, such gas phase atomic or molecularspecies 120 may be formed by heating and evaporating a material using avariety of known power sources, including a resistive or radiativeheater or an electron beam. The PLD technique is similar, but irradiatesa material using a pulsed laser to generate gas phase atomic ormolecular species 120. In the sputtering technique, such gas phaseatomic or molecular species 120 may be formed by bombarding a materialwith energetic particles so as to liberate molecules of the materialinto the gas phase; the energetic particles may be generated using avariety of known sources, including a plasma, an ion source, a particleaccelerator, or a radioactive material. In the CVD and ALD techniques,such gas phase atomic or molecular species 120 may be stored separatelyin gaseous form and introduced to a reaction chamber that contains thesubstrate; the species optionally may be activated using a suitablesource, such as with a plasma, combustion, or thermal decomposition.

As illustrated in FIG. 1A, gas phase atomic or molecular species 120 mayadsorb onto substrate 110, forming adsorbed atomic or molecular species120′. Upon such adsorption, atomic or molecular species 120′ may form amolecular bond with substrate 110, e.g., a covalent or ionic bond, or abond based on dipole-dipole interactions, London dispersion force, orhydrogen bonding. Upon such bonding with substrate 110, atomic ormolecular species 120′ may directly form material 140 disposed onsubstrate 110, as illustrated in FIG. 1B, or may first undergo a furtherchemical reaction, e.g., with substrate 110, with another atomic ormolecular species 120′ adsorbed to substrate 110, or with another gasphase atomic or molecular species 120, to form material 140 illustratedin FIG. 1B. In many situations, the adsorbed atomic or molecular species120′ may move along the surface to find the right accommodation (e.g.,site on substrate 110, or another gas phase or adsorbed atomic ormolecular species) for the ultimate reaction to occur. At themacroscopic level, the processing parameters used during a givenmaterial deposition technique may include the type and power level ofany source selected to assist with generating or activating gas phaseatomic or molecular species 120; any electrical bias that may be appliedto substrate 110; any heating or cooling that may be applied to gasphase atomic or molecular species 120 or to substrate 110; the flow rateor concentration of gas phase atomic or molecular species 120; thepressure or partial pressure of gas phase atomic or molecular species120; and the amount of time with which substrate 110 is exposed to gasphase atomic or molecular species 120.

At the microscopic level, the energy barrier that may be required toconvert gas phase atomic or molecular species 120 illustrated in FIG. 1Ainto material 140 illustrated in FIG. 1B may be far lower than the totalamount of energy provided by the processing parameters. In this regard,at the microscopic level, substrate 110 may be viewed as an energy“sink” toward which atomic or molecular species 120 is attracted, andadditional energy may be applied to “agitate” adsorbed atomic ormolecular species 120′—that is, to enhance the mobility of species 120on substrate 110—to convert species 120′ into material 140. Theconversion of species 120 to material 140 thus may be considered to havetwo different types of energy deficits, the first arising from theparticular chemical reactivity of atomic or molecular species 120, andthe second being of a more general thermodynamic and kinetic nature.

The processing parameters may nominally provide the amount of energythat may be required at the microscopic level to overcome both the firstand second types of energy deficits. However, such processing parametersmay be applied at the macroscopic or “bulk” level and thus applied tothe entirety of substrate 110 and to all gas phase species 120 and alladsorbed species 120′. For example, as illustrated in FIGS. 1A-1B,substrate 110 may be disposed on heater 130 that heats substrate 110 toa temperature sufficient to enhance the mobility of absorbed species120′ to convert such species into material 140. However, such bulkheating may produce a number of engineering constraints that may limitthe type of material 140 that may be deposited on a particular substrate110. Specifically, the material from substrate 110 is made, includingthe materials of any structures buried therein, must be compatible withthe bulk heating temperature used; while the substrate is maintained atthe bulk heating temperature, other objects in the reaction chamberpreferably are kept sufficiently cool to inhibit contamination; andsupporting hardware to maintain substantially uniform heating, cooling,or temperature stability must be provided to maintain uniform growth ofmaterial 140 on substrate 110. In particular, elevated temperatures maycause materials buried within substrate 110 to diffuse into each otheror into substrate 110, thus damaging the materials.

Thus, what is needed is a way to enhance mobility of adsorbed atomic ormolecular species on a substrate, while reducing the bulk temperature ofthe substrate.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide systems and methods forenhancing mobility of atomic or molecular species on a substrate atreduced bulk temperature using acoustic waves, and structures formedusing the same. In particular, the energy from a pulsed laser, undercontrolled parameters, may be converted into acoustic waves thatpropagate across a substrate and enhance molecular mobility at distanceremoved from the source of the acoustic waves. With this ability,embodiments of the invention allow for the development of a non-contact,non-intrusive method for preparing materials that may be adapted for usewith existing materials processing reaction chambers. The enhancedmobility induced by acoustic waves additionally, or alternatively, maybe used to remove atomic or molecular species adsorbed on a substratesurface as a way of “cleaning” or sterilizing the substrate or toinhibit specific molecular species from ever residing on a surface, orto gather or pattern adsorbed atomic or molecular species to one or moreselected locations on a surface.

Under one aspect of the present invention, a method for enhancingmobility of an atomic or molecular species on a substrate may includeexposing a first region of a substrate to an atomic or molecular speciesthat forms a molecular bond with the substrate in the first region;directing a laser pulse to a second region of the substrate so as togenerate an acoustic wave in the second region, the acoustic wave havingspatial and temporal characteristics selected to alter the molecularbond; and transmitting the acoustic wave from the second region to thefirst region, the acoustic wave altering the molecular bond between thesubstrate and the atomic or molecular species to enhance mobility of theatomic or molecular species on the substrate in the first region.

Responsive to the altering of the molecular bond, the atomic ormolecular species may translate (e.g., hop or jump) laterally across thesubstrate in a direction defined by the spatial and temporalcharacteristics of the acoustic wave. Alternatively, or additionally,responsive to the altering of the molecular bond, the atomic ormolecular species may form a material (e.g., may form nanometer sheetsof graphene films by epitaxial growth). Alternatively, responsive to thealtering of the molecular bond, the atomic or molecular species maydissociate (e.g., shed atoms) to form more chemically energetic species,such as radicals, that induce surface chemical reaction (e.g., formationof graphene by methane/ethane carbon source adsorption). The substratemay have a damage threshold temperature and wherein in the absence ofthe acoustic wave, the atomic or molecular species may form the materialor otherwise diffuse only at a reaction temperature that is higher thanthe damage threshold temperature of the substrate. For example, thesubstrate may include an integrated circuit, a chalcogenide glass, aZBLAN glass, or a polymer such as polycarbonate, poly(methylmethacrylate), polystyrene, polyvinyl chloride, or polyethyleneterephthalate. The material may include silicon nitride, graphene,carbon nanotubes, diamond, titanium dioxide, titanium boride, zirconiumoxide, yttria-stabilized zirconium, boron carbide, boron nitride, ormetal. Alternatively, there may be the desire to form a material (e.g.,graphene) using a convenient molecular source (e.g., methane) but atmuch lower temperatures (e.g., currently ˜800 C. or above may berequired).

In some embodiments, the laser pulse has a temporal duration of lessthan about 1 nanosecond at FWHM, or 1 picosecond at FWHM, or less thanabout 100 femtoseconds at FWHM. The laser pulse may be focused to apoint in the second region of the substrate and the acoustic wave istransmitted radially from the point. Alternatively, the laser pulse maybe focused to a line in the second region of the substrate and theacoustic wave is transmitted linearly from the line as a plane wave.Alternatively, the laser pulse may be focused to form a pattern in thesecond region of the substrate so as to generate a complex radiatingacoustic pattern.

In some embodiments, the acoustic wave includes a Rayleigh wave, whichmay have a bandwidth of 100 MHz at FWHM or more.

The molecular bond may include a covalent bond, an ionic bond, or a bondbased on dipole-dipole interactions, London dispersion force, orhydrogen bonding.

Under another aspect of the present invention, a structure may include asubstrate having a damage threshold temperature; and a material disposedon the substrate, the material having a reaction temperature that ishigher than the damage threshold temperature of the substrate.

The substrate may include an integrated circuit, a chalcogenide glass, aZBLAN glass, or a polymer such as polycarbonate, poly(methylmethacrylate), polystyrene, polyvinyl chloride, or polyethyleneterephthalate. The material may include silicon nitride, graphene,carbon nanotubes, diamond, titanium dioxide, titanium boride, zirconiumoxide, yttria-stabilized zirconium, boron carbide, boron nitride, ormetal.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1B schematically illustrate structures that may be formedduring a previously known method for preparing a material.

FIGS. 2A-2D schematically illustrate structures that may be formedduring a method for preparing a material at reduced bulk temperature byenhancing mobility of molecular species with an acoustic wave, accordingto one exemplary embodiment of the present invention.

FIG. 3 illustrates steps in a method enhancing mobility of molecularspecies with an acoustic wave, according to one exemplary embodiment ofthe present invention.

FIG. 4 illustrates damage threshold temperatures for selected substratesand reaction temperatures for selected materials.

FIG. 5 illustrates a system for preparing a material at reduced bulktemperature by enhancing mobility of molecular species with an acousticwave, according to one exemplary embodiment of the present invention.

FIGS. 6A-6D are microscopic images of gold species disposed on asubstrate under different focus and displacement conditions, accordingto one exemplary embodiment of the present invention.

FIG. 7A illustrates excitation (dashed) and emission (solid) spectra ofdifferent gold molecular species.

FIGS. 7B-7C are microscopic images of gold molecular species disposed ona substrate under different optical filters, according to one exemplaryembodiment of the present invention.

FIGS. 8A-8B are microscopic images of a gold molecular species disposedon a substrate before and after repeated exposure to acoustic waves,respectively, according to one exemplary embodiment of the presentinvention.

FIGS. 8C-8E schematically illustrate the measured travel of a goldmolecular species across a substrate during repeated exposure toacoustic waves generated with laser pulses of different energies thanone another, according to exemplary embodiments of the presentinvention.

FIGS. 9A-9B schematically illustrate the measured travel of a goldspecies across a substrate during a first period of repeated exposure toacoustic waves and a second period of no exposure to acoustic waves,according to one exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide systems and methods forenhancing mobility of molecular species on a substrate at reduced bulktemperature using acoustic waves, and structures formed using the same.In particular, embodiments of the present invention may provide systemsand methods of preparing materials or processing substrates by enhancingthe mobility or diffusion of molecular species on a substrate surface byirradiating the substrate with a relatively short laser pulse togenerate an acoustic wave that then alters a molecular bond, e.g.,alters the strength of a molecular bond, between the atomic or molecularspecies and substrate. Such alteration enhances the mobility of theatomic or molecular species on the substrate by reducing chemicalbonding barriers that may entrap an atomic or molecular species at aspecific site on the substrate. For example, responsive to thealteration of the molecular bond, the atomic or molecular species maytranslate laterally across the substrate in a direction defined by thespatial and temporal characteristics of the acoustic wave, which in turnare defined by the spatial and temporal characteristics of the laserpulse. Or, for example, responsive to the alteration of the molecularbond, the atomic or molecular species may chemically react to form amaterial. As such, by enhancing mobility of the atomic or molecularspecies on the substrate using acoustic waves, the temperature of thesubstrate may be reduced relative to what otherwise may have beenrequired to achieve comparable mobility via bulk heating.

Accordingly, some embodiments of the present invention may allowmaterials that are technologically important, but that have relativelyhigh reaction temperatures, to be deposited on substrates that havedamage threshold temperatures that are lower than such reactiontemperatures. Examples of such materials may include silicon nitride(SiN_(x)), graphene, carbon nanotubes, diamond, titanium dioxide (TiO₂),titanium boride (TiB₂), zirconium oxide (ZrO₂), yttria-stabilizedzirconium (YSZ), boron carbide (B₄C), boron nitride (BN), or a metal,and examples of such substrates may include integrated circuits,chalcogenide glasses, fluoride glasses such as ZrF₄—BaF₂—LaF₃—AlF₃—NaF(ZBLAN) glasses, or polymers, e.g., polycarbonate, poly(methylmethacrylate), polystyrene, polyvinyl chloride, or polyethyleneterephthalate. Or, for example, some embodiments of the presentinvention may allow undesired atomic or molecular debris to be cleaned,removed, or directed from a substrate surface, by using the acousticwaves to enhance diffusion of such debris on the surface.

First, some exemplary structures that may be formed using the presentinvention, and an illustrative method for forming same, will beprovided. Then, an exemplary system for implementing such a method willbe described. Lastly, several experiments that have been performed thatdemonstrate enhanced molecular mobility will be described.

FIGS. 2A-2D illustrate structures that may be formed during steps ofsome embodiments of the inventive method. Specifically, as illustratedin FIG. 2A, substrate 210 may be exposed to an atomic or molecularspecies 220 that is in the gas phase, and that may be generated by anysuitable technique, e.g., any PVD or CVD process that is previouslyknown, such as those described above, or that is yet to be developed.Gas phase atomic or molecular species 220 may adsorb onto substrate 210,forming adsorbed atomic or molecular species 220′. Upon such adsorption,atomic or molecular species 220′ may form a molecular bond withsubstrate 210, e.g., a covalent or ionic bond, or a bond based ondipole-dipole interactions, the London dispersion force, or hydrogenbonding. Such a molecular bond may be distinguished from bonds that mayform between larger (non-molecular) particles and substrate 210, e.g.,particles that have greater than about 10 atoms, or greater than about100 atoms, or greater than about 1000 atoms, which instead may be basedon space-charge or coulombic interactions.

As illustrated in FIG. 2A, a first portion of substrate 210 may beirradiated with laser pulse 230, which may have a set of preselectedcharacteristics including a set of optical wavelengths λ_(L), a temporalduration τ_(L), a spatial profile Λ_(L), and an intensity I_(L,). Inembodiments in which the laser pulse is optically, temporally, orspatially Gaussian, such parameters may be described by their full widthof the pulse at half of the pulse's maximum (FWHM). In embodiments inwhich the laser pulse has more complex features, e.g., contains opticalwavelengths that are not contiguous with one another, contains multiplediscrete temporal features, or has multiple discrete spatial features,such parameters may be described as appropriate. Additionally, the laserrepetition rate R(t), the laser pulse amplitudes A(t), or the burst rate(number of laser shots per burst) B(t), each may be individually variedto enhance the generation of the acoustic wave while inhibiting damageto substrate 210. Preferably, the set of optical wavelengths λ_(L) oflaser pulse 230 is at least partially absorbed by substrate 210, andthus heats the irradiated first portion of substrate 210. In someembodiments, the first portion of substrate 210 may include a coating(not illustrated) having optical characteristics selected to at leastpartially absorb the set of optical wavelengths λ_(L) of laser pulse230.

Preferably, the temporal duration τ_(L) of laser pulse 230 is relativelyshort so as to cause sufficiently rapid thermal expansion of substrate210 as to generate acoustic waves 250 illustrated in FIG. 2B. Forexample, laser pulse 230 may have a substantially Gaussian temporalprofile, and may have a temporal duration of less than about 10nanoseconds at FWHM, or less than about 1 nanosecond at FWHM, or lessthan about 100 picoseconds at FWHM, or less than about 10 picoseconds atFWHM, or less than about 1 picosecond at FWHM, or less than about 100femtoseconds at FWHM, in one example about 8-10 nanoseconds at FWHM.Additionally, the spatial profile Λ_(L) of laser pulse 230 is relativelysmall in at least one dimension, such that the acoustic wave has asufficiently small spatial profile in that dimension to sufficientlyalter the molecular bond between adsorbed atomic or molecular species220′ and substrate 210, and thus to enhance mobility of atomic ormolecular species 220′ on substrate 210. For example, laser pulse 230may be focused to a point, to a line, or to a more complicated pattern,as appropriate. In some embodiments, the spatial profile Λ_(L) of laserpulse 230 may have a dimension of about 20 microns or less in at leastone dimension, or less than about 10 microns or less in at least onedimension, or less than about 5 microns or less in at least onedimension, or less than about 2 microns or less in at least onedimension. Preferably, the intensity I_(L) of laser pulse 230 isselected so as to reduce or inhibit damage to substrate 210, but at thesame time to generate acoustic wave having sufficient amplitude, incombination with the other acoustic wave parameters, to alter molecularbonds between adsorbed atomic or molecular species 220′ and substrate210. To mitigate local damage that may be induced by the laser, thelaser pulse 230 may be repeatedly moved (scanned) across the surface ofsubstrate 210, e.g., using reflecting mirror 590 described in greaterdetail below with reference to FIG. 5. In one embodiment, the intensityI_(L) of laser pulse 230 is selected so as to induce a thermoelasticmaterial response in substrate 210. Laser pulse 230 may be generated byany suitable pulsed laser, such as an diode pumped, Q-switched,Nd-Vanadate ultrafast laser, or a diode pumped, cavity dumped,Nd-Vanadate ultrafast laser or a flashlamp pumped Nd-YAG nanosecondpulse laser.

It should be appreciated that the spatial profile Λ_(ac) and intensityI_(ac) of the acoustic wave depend not only on the spatial profile Λ_(L)and intensity I_(L) of laser pulse 230, but also on the temporalduration τ_(L) of the laser pulse, the thermal expansion coefficient ofsubstrate 210, and the rate at which substrate 210 expands responsive todeposition of heat by the laser pulse. Specifically, as illustrated inFIG. 2B, the spatial profile Λ_(th) of heated region 240 may be definedby, and in some embodiments may be substantially the same as, thespatial profile Λ_(L) of laser pulse 230. The spatial profile Λ_(ac) ofthe acoustic waves 250 resulting from rapid thermal expansion ofsubstrate 210 within heated region 240 may be defined by, and in someembodiments may be substantially the same as, the spatial profile Λ_(th)of heated region 240. However, if temporal duration τ_(L) of laser pulse230 is too long, then the laser pulse will not cause heated region 240of substrate 210 to thermally expand with sufficient rapidity togenerate acoustic waves 250. Waves 250 have a temporal profile τ_(ac)based on the waves' respective spatial profiles Λ_(ac) and the speed ofsound ν in the material for waves having such a spatial profile, and abandwidth that is inversely proportional to the temporal profile τ_(ac).Acoustic waves 250 may include surface waves or bulk waves (e.g.,longitudinal or transverse waves or harmonics thereof, or combinationsthereof), as denoted by the dashed arrows within substrate 210.Preferably, acoustic waves 250 have a bandwidth of at least 100 MHz atFWHM, or at least 250 MHz at FWHM, or at least 1 GHz at FWHM, or atleast 100 GHz at FWHM, or at least 1 THz or more.

As is known in the art, acoustic waves conform to mathematical analysisby the wave equation, and consequently have attributes common to wavesin general, such as wave interference. In preferred embodiments,acoustic waves 250 include one or more modes that are substantiallyconfined at the interface between substrate 210 and gas phase atomic ormolecular species 220. For example, a combination of compression (e.g.,longitudinal) and shear modes (e.g., transverse) may lead to a surfacewave that may propagate along an interface between a gas and a solid,with the Rayleigh mode being one that is mathematically tractable, as isknown in the art. A surface wave typically penetrates into the solidmedium, e.g., into substrate 210, by a distance equivalent toapproximately 1-2 wavelengths. As such, an acoustic wave having afrequency of 100 MHz and a wavelength of 50 microns, traveling along thesurface of an exemplary substrate having an acoustic velocity ν of 5000m/s, may penetrate approximately 100 microns into the substrate.Preferably, the acoustic wave is “broadband,” that is, includes manyfrequencies and has a relatively high bandwidth, rather than“narrowband,” e.g., containing only a single frequency (such as 100 MHz)and having a relatively low bandwidth. In this regard, a narrowbandsingle-frequency acoustic wave traveling at a solid-gas interface may bemathematically shown to only displace atoms or molecules at theinterface in an elliptical pattern, but without inducing net lateraltranslational motion. However, it is believed that a broadband acousticwave may induce net lateral translational motion of atoms or moleculesat such an interface, thus enhancing mobility of those atoms ormolecules.

As illustrated in FIG. 2C, acoustic wave 250 is transmitted along thesurface of substrate 210 from a transient heated region 240 to a secondregion in which adsorbed molecular species 220′ is molecularly bonded tosubstrate 210. Here, acoustic wave 250 alters the molecular bond betweenthe adsorbed atomic or molecular species and substrate 210, e.g., altersthe strength of the molecular bond, as indicated by the deformation inadsorbed atomic or molecular species 220″. Such alteration enhances themobility of adsorbed atomic or molecular species 220″ without requiringa corresponding increase in the bulk temperature of substrate 210, aswas previously known. Note that such enhanced mobility would beunexpected based only on the equilibrium energetics of the system.Specifically, the energy of acoustic wave 250 may be on the order of amillielectron volt (meV). The energy of the molecular bond betweenatomic or molecular species 220″ and substrate 210 may be on the orderof hundreds of meV if the bond is based on physical adsorption, e.g., abond based on dipole-dipole interactions or the London dispersion force,or may be on the order of an eV if the bond is based on chemicaladsorption, e.g., a covalent bond, an ionic bond, or a hydrogen bond.Accordingly, it would be unexpected that an acoustic wave having anenergy that is orders of magnitude lower than the bond energy would beable to alter the molecular bond between atomic or molecular species220″ and substrate 210, particularly to an extent that would enhancemobility of the atomic or molecular species.

However, without wishing to be bound by any theory, it is believed thatan acoustic wave propagating at the surface of substrate 210, and havingsufficient intensity and bandwidth, may induce a transient strain atthat surface, and thus may momentarily alter the strength of the bondbetween atomic or molecular species 220″ and substrate 210. For example,an acoustic wave having a 100 MHz bandwidth in a substrate having anacoustic velocity of 5000 m/s may interact with atomic or molecularspecies 220″ for approximately 10 nanoseconds; it will be appreciatedthat the duration of such interaction may vary based on the acousticvelocity of the particular substrate being used, as well as the spatialprofile Λ_(ac) and temporal profile τ_(ac) of the acoustic wave.

As noted above, acoustic wave 250 preferably is a broadband acousticwave. It is believed that the various frequency components of acousticwave 250, and in particular the high frequency components, may induce anet directed force in a lateral direction on adsorbed atomic ormolecular species 220″. On the microscopic scale, the transient wave mayinduce surface strain of local substrate atoms to which the atomic ormolecular species 220″ is molecularly bonded. The local strain may havethe effect of reducing the height of the barrier that has immobilizedthe atomic or molecular species. The adsorbed atomic or molecularspecies 220″ is always in random motion as consequence of localtemperature (e.g., an effect known as Brownian motion in liquids). Thelowering of the barrier height may provide a path along which thedirected force of the acoustic wave 250 may act. Responsive to thecombination of such a net directed force and the alteration in strengthof the molecular bond concurrently induced by acoustic wave 250, atomicor molecular species 220″ may have enhanced mobility, e.g., may movelaterally along the surface of substrate 210 to which it is adsorbed. Itshould be appreciated that the magnitude of such enhanced mobility maydepend upon the parameters of acoustic wave 250 and the nature of themolecular bond between atomic or molecular species 220″ and substrate210. Additionally, as acoustic wave 250 propagates along substrate 210,acoustic wave 250 also may interact with additional adsorbed atomic ormolecular species 220′, thus enhancing the mobility of such species aswell. However, higher frequency components of the acoustic wave may dampas the wave propagates (as consequence of dispersion properties of thematerial from which substrate 210 is formed to acoustic wavepropagation), with a rate proportional to the inverse of the square ofthe frequency.

The distance traveled by adsorbed atomic or molecular species 220″responsive to a single acoustic wave 250 may be relatively small, e.g.,on the order of about one tenth of a length of the bond between atomicor molecular species 220″ and substrate 210, e.g., on the order of about0.001 nm in one example, or on the order of about 0.01 nm in anotherexample, or on the order of about 0.1 nm in another example, or on theorder of about 1 nm in another example, or on the order of about 10 nmin still another example. So as to increase the net distance traveled byatomic or molecular species 220″, substrate 210 may be irradiated by aplurality of laser pulses 230 at a predetermined laser repetition rateR(t), amplitude A(t), and burst rate B(t), each laser pulse generatingan acoustic wave 250 that moves species 220″ an additional distance.Preferably, the repetition rate R(t) is selected such that at least someof the heat deposited by each laser pulse 230 dissipates into substrate210 before the next laser pulse arrives, so as to reduce the buildup ofheat in substrate 210; the use of a laser shot in short burst modes mayfurther mitigate thermal buildup; the dissipation rate of such heat maydepend on the thermal conductivity κ of substrate 210. Examples ofsuitable repetition rates R(t) may be in the range of kHz, tens of kHz,hundreds of kHz, or even MHz or greater. Optionally, laser pulses 230may be applied in “bursts” with a burst rate B(t) selected to permitheat to sufficiently diffuse out of heated region 240 between bursts,e.g., using a fast optical shutter. In one example, burst rate B(t) maybe on the order of 1000 laser shots or less, or 10,000 laser shots orless. In addition, the laser pulse amplitudes A(t) in the burst may alsobe altered at a single pulse unit to further allow diffusion of heat.

Note that each acoustic wave 250 in a plurality of acoustic waves 250need not necessarily be generated at the same physical region ofsubstrate 210. Instead, each subsequent laser pulse 230 optionally mayirradiate different regions of substrate 210, with correspondingacoustic waves 250 being generated by rapid thermal expansion of suchregions. In one illustrative embodiment, a moving mirror, such as mirror590 illustrated in FIG. 5, or other optical element may be used todynamically direct different laser pulses 230 to different regions ofsubstrate 210. Additionally, each acoustic wave 250 in a plurality ofacoustic waves need not necessarily be generated by a single laser.Instead, one or more laser pulses 230 may be generated by a first laser,and one or more laser pulses may be generated by one or more additionallasers. Moreover, each laser pulse 230 may be spatially and/ortemporally shaped using known techniques such that the correspondingacoustic waves 250 have appropriate characteristics to suitably alterthe molecular bond between adsorbed atomic or molecular species 220″ andsubstrate 210. Indeed, a sequence of laser pulses 230 from one or morelasers may be tailored such that the corresponding acoustic waves 250interfere with one another in such a manner as to produce a specificspatial excitation mode that enhances mobility of adsorbed atomic ormolecular species, while reducing the presence of waves that do notenhance mobility of adsorbed atomic or molecular species, such asstanding waves. In some embodiments, the laser pulse(s) are spatiallyand/or temporally shaped so as to direct adsorbed atomic or molecularspecies into a desired pattern on substrate 210.

For example, in embodiments where laser pulse 230 is focused to a point,acoustic wave 250 may propagate radially away from that point and intothe bulk as well. In embodiments where laser pulse 230 is focused to aline, acoustic wave 250 may propagate as a planar surface wave linearlyaway from that line. The former wave shape may be used to clear adsorbedatomic or molecular species away from an area, while the latter may beused to induce motion in a particular direction. Other spatial profilesfor laser pulse 230 are possible. For example, a laser beam may be splitto form multiple laser pulses 230 that are individually focused to aline or a point, but that irradiate substrate 210 in close proximity toone another. Such a technique may narrow the frequency content of theresulting acoustic wave, through interference phenomenon, and may besuitable for use in moving larger atomic or molecular species. Or, forexample, two pulsed lasers that are synchronized to one another may beused to irradiate substrate 210 with different spatial profiles than oneanother, e.g., so as to generate a first acoustic wave with a relativelylow bandwidth (e.g., 10 MHz) and a second acoustic wave with arelatively high bandwidth (e.g., 100 MHz). The sum of the two acousticwaves may be used to alter molecular bonds between adsorbed atomic ormolecular species 220″ and substrate 210.

Responsive to the mobility enhancement induced by one or more acousticwaves 250, adsorbed atomic or molecular species 220″ may directly formmaterial 260 disposed on substrate 210, as illustrated in FIG. 2D, ormay first undergo a further chemical reaction, e.g., with substrate 210,with another atomic or molecular species 220′ adsorbed to substrate 210,or with a gas phase atomic or molecular species 220, to form material260 illustrated in FIG. 2D.

Now referring to FIG. 3, an exemplary method 300 of enhancing mobilityof an atomic or molecular species with an acoustic wave, according toone exemplary embodiment of the present invention. Method 300 includesco-selecting a substrate, atomic or molecular species, and processparameters based on the material to be prepared (step 310).Specifically, the process parameters may include laser parameters suchas the laser pulse wavelength λ_(L), temporal profile τ_(L), spatialprofile Λ_(L), and intensity I_(L), which may be selected as describedabove, as may any other process parameters such as a technique forgenerating the atomic or molecular species (e.g., PVD or CVD basedtechnique such as described above or known in the art), the pressure orpartial pressure and flow rate of the atomic or molecular species, andthe bulk temperature of the substrate.

Then, the selected substrate is provided and prepared (step 320), forexample using any suitable technique known in the art. For example, theupper surface of the substrate may be suitably cleaned in preparationfor forming a material thereon. The substrate may include one or moreadditional layers therein, including insulators, conductors, and/orsemiconductors. In one embodiment, the substrate is a “low temperaturesubstrate,” by which it is meant a substrate having a damage thresholdtemperature that is lower than the reaction temperature of the materialto be deposited therein. Examples of such substrates may include, forexample, an integrated circuit, a chalcogenide glass, a ZBLAN glass, ora polymer such as polycarbonate (PC), poly(methyl methacrylate) (PMMA),polystyrene (PS), polyvinyl chloride (PVC), or polyethyleneterephthalate (PET).

Then, a first region of the substrate is exposed to a gas phase atomicor molecular species (step 330). Alternatively, the atomic or molecularspecies may be dissolved in liquid, such as a high vapor pressuresolvent (e.g., methanol, ethanol, acetone), and the solvent thenevaporated. Responsive to such exposure, the atomic or molecular speciesmay adsorb on the substrate. The gas phase atomic or molecular speciesmay be generated using physical vapor deposition (PVD) based techniquessuch as evaporation, sputtering, molecular beam epitaxy (MBE), andpulsed laser deposition (PLD), or chemical vapor deposition (CVD) basedtechniques, including atomic layer deposition (ALD), or a liquid basedsystem. In some embodiments, the gas phase atomic or molecular speciesis a precursor to a technologically important material such as siliconnitride, graphene, carbon nanotubes, diamond, titanium dioxide, titaniumboride, zirconium oxide, yttria-stabilized zirconium, boron carbide,boron nitride, or metal. As illustrated in FIG. 4, the reactiontemperature of such materials may be significantly higher than thedamage threshold temperature of low temperature substrates, e.g.,polymers such as polycarbonate (PC), poly(methyl methacrylate) (PMMA),polystyrene (PS), polyvinyl chloride (PVC), or polyethyleneterephthalate (PET). It may not be practicable to deposit such materialson low temperature substrates using bulk heating, because the substratewould need to be heated above its damage threshold temperature toachieve the reaction temperature of the material. Instead, enhancing themobility of precursors to such materials on the surface of such asubstrate using acoustic waves may reduce the bulk temperature to whicha substrate need be heated to form the material. Alternatively, afterthe region is exposed to a gas phase or molecular species, the surfacemay be exposed to photons (e.g., such as generated by lasers, lamps, orX-ray sources), or to electrons, responsive to which the surface layermay be altered so as to allow a more efficient use of the acoustic wavesto enhance surface mobility, e.g., to induce material growth or toremove atomic or molecular species from the substrate.

Note that during step 330 illustrated in FIG. 3, the entire uppersurface of the substrate may be exposed to the gas phase atomic ormolecular species. However, the term “first region” is intended todistinguish the region of the substrate in which molecular bonds are tobe altered with an acoustic wave, from the “second region” of thesubstrate to be irradiated with a laser pulse so as to generate such anacoustic wave during step 340. The first and second regions may beseparated from one another by any desired distance, e.g., about 10microns or more, or about 100 microns or more, or about 1 millimeter ormore, or about 10 millimeters or more, or about 100 millimeters or more,or about 1 centimeter or more.

Then, the mobility of atomic or molecular species adsorbed to thesubstrate is enhanced using acoustic waves (step 340). Specifically, inthe embodiment illustrated in FIG. 3, a laser pulse such as describedherein is directed to a second region of the substrate (step 341). Thelaser pulse generates a corresponding acoustic wave such as describedherein, which has spatial and temporal characteristics selected to altera molecular bond between the adsorbed atomic or molecular species andthe substrate (step 342). The acoustic wave is then transmitted from thesecond region to the first region of the substrate so as to alter themolecular bond between the adsorbed atomic or molecular species and thesubstrate, thus enhancing the mobility of the atomic or molecularspecies as described herein (step 343).

Responsive to the alteration of the molecular bond between the adsorbedatomic or molecular species and the substrate, the atomic or molecularspecies may form a material such as described herein (step 350). In onespecific example, the atomic or molecular species includes carbon thatforms a graphene film by epitaxial growth. In another specific example,the substrate includes a metal film catalyst that has been impregnatedwith the atomic or molecular species, and responsive to the alterationof the molecular bond, the atomic or molecular species provides a seedfor growth of material. Alternatively, the atomic or molecular speciesmay translate across the substrate in a direction defined by theacoustic wave (step 360). In another alternative embodiment (notillustrated), the atomic or molecular species may dissociate (e.g., shedatoms) to form a more chemically energetic species, such as a radical.Such energetic species may induce a surface chemical reaction, such asthe formation of graphene by methane/ethane carbon source adsorption.

FIG. 5 schematically illustrates a system 500 for use in enhancing themobility of molecular species on a substrate, according to one exemplaryembodiment of the present invention. System 500 includes controller 510,stage 520, laser 530, database 540, reaction chamber 580, and mirror590. Reaction chamber 580 includes an optical window (not shown) viawhich the laser beam may irradiate the interior of the chamber.Controller 510 is in operable communication with stage 520, laser 530,database 540, reaction chamber 580, and mirror 590 (communication withmirror 590 not shown). The system also may include optical modulator 531disposed along the optical path between laser 530 and reaction chamber580. Preferably, optical modulator 531 includes an electro-optic fastshutter system or pulse slicer configured to modify the laser pulseamplitudes A(t) for each shot, the burst rate B(t), and the laser pulsetime profile, and is controlled by the processor/controller 510.Controller 510 includes memory 550 (e.g., a computer-readable medium)configured to store processing instructions, processor 560 configured tostore the stored processing instructions, display device 511 configuredto display data to a user, and input device 512 configured to acceptinput from a user. Database 540 contains information on how to prepare avariety of different types of materials, or otherwise process asubstrate, by enhancing mobility of molecular species. Database 540 maybe integral to controller 510, or may be remote to controller 510 and inoperable communication with controller 510 via a network, such as theInternet.

Stage 520 is positioned within reaction chamber 580 and supportssubstrate 210, and is operable to adjust the position of the substratein the x, y, and z directions responsive to instructions from controller510. Alternatively, mirror 590 may include a high speed scanning mirrorwith a compensating Z-motion focus unit that is configured to move thelaser beam while maintaining substrate 210 in a stationary position.During execution of step 340 of FIG. 3, laser 530 emits light of awavelength λ_(L) selected to be absorbed by substrate 310 so as torapidly heat an irradiated region of substrate 310, and thus generate anacoustic wave. In other embodiments (not shown) one or more additionallasers may be provided to heat substrate 310 so as to generate acousticwaves.

Mirror 590 directs the light from laser 530 toward one or more regionsof substrate 210 in accordance with instructions from controller 510,preferably through a window or port-hole in reaction chamber 580. In analternative embodiment (not shown), stage 520 is used to move substrate210 relative to the laser beam, instead of using mirror 590 to directthe light to different portions of substrate 210. For example, forrelatively large reaction chambers (e.g., chambers configured toaccommodate one or more substrates having a collective area of one ormore square meters, or ten or more square meters, such as industrialautoclaves) it may be preferable to move the laser beam using mirror590, rather than moving stage 520. Reaction chamber 580 is configured tomaintain substrate 210 at a selected pressure or partial pressure ofmolecular species generated by atomic or molecular species source 581during the appropriate processing time, as well as at a selected bulktemperature (which may be significantly lower than the bulk temperaturethat may be required in the absence of the acoustic waves). Note that ifsubstrate 210 is to be exposed to different types of atomic or molecularspecies, then one or more corresponding atomic or molecular speciessources 581 may be provided and configured to provide such species toreaction chamber 580 as appropriate under control of controller 510.

Responsive to user input provided through input device 512, e.g., userinput defining the material to be prepared on substrate 210 or otherprocessing to be performed on substrate 210, controller 510 requestsdatabase 540 to provide information on how to prepare that type ofmaterial or to perform such processing. Responsive to the request,database 540 provides some or all of the following information tocontroller 510: the type of substrate 210 to be used; any requiredpreparation thereof; the wavelength(s) and other parameters of laserlight to be used; the type of atomic or molecular species source to beused and any parameters thereof, including the pressure or partialpressure of the atomic or molecular species; and any additionalprocessing to be performed after exposing the substrate 210 to theatomic or molecular species and laser light. Controller 510 receivesthis information and stores it in memory 550. Processor 560 processesthe stored information, and based on that information displaysinstructions to the user via display device 511 and controls stage 520,laser 530, reaction chamber 580, and mirror 590 to process the substrate210 as appropriate.

In one example, the user uses input device 512, e.g., a keyboard andmouse, to input to the controller that he desires to prepare aparticular material disposed on a particular substrate, e.g., a TiO₂film on a PMMA substrate. Responsive to that input, controller 510requests database 540 to provide information on preparing such astructure. Responsive to the request, database 540 provides a set ofinstructions to the controller 510, which controller 510 stores inmemory 550. Processor 560 then processes the stored instructions todetermine what information is to be displayed to the user via displaydevice 511, and how the various components of the system are to becontrolled. For example, processor 560 may cause display device 511 todisplay the type of substrate and molecular species to be used so thatthe user may obtain and prepare the substrate and molecular species, tothe extent that system 500 is not configured to obtain the substrate andmolecular species automatically (without further user intervention).

Next, the user places the prepared substrate 210 on stage 520 and usesinput device 512 to inform controller 510 that the substrate is ready.Responsive to this input, processor 560 instructs stage 520 to move to afirst pre-determined position in the x, y, and z directions andinstructs reaction chamber 580 to expose substrate 310 to an atomic ormolecular species from molecular species source 581 at a suitablepressure or partial pressure, based on the stored instructions.Processor 560 then instructs laser 530 to emit light having wavelengthλ_(L) and other parameters such as described herein, and instructsmirror 590 to guide that light to the appropriate region(s) of substrate210. The light then generates an acoustic wave that enhances themobility of molecular species adsorbed to substrate 210.

Those of skill in the art will appreciate that any of the user-performedsteps may alternatively be automated using commercially availableequipment (not illustrated). For example, in certain industrialapplications, a process script profile may be provided that controlsgases, laser, motion and timing. For example, instead of displaying tothe user what type of substrate and film is to be provided, controller510 may instead be in operable communication with a robotic substratehandler that may obtain substrate 210 from a substrate store, and mayprocess the substrate as appropriate. In one example, the substrate maybe automatically dispensed e.g., using a tape dispensing roller. In oneembodiment, one or more steps of an instruction sequence are madecontingent on a feedback parameter, such as a spectrum of lightreflected from the substrate, or a change in reflectivity of thesubstrate resulting from deposition of material.

For example, a pulsed laser beam from a separate probe laser (not shown)may be used to periodically irradiate a region of the substrate 210where a material is being deposited, and a reflected portion of theprobe beam then input into a photodetector (also not shown) incommunication with controller 510. The output of the photodetector maybe analyzed to determine whether a material had been deposited on theregion of the substrate, and if so, how much. For example, controller510 may include software stored in memory 550 operable on processor 560for determining, based on the photodetector output, whether thereflectivity of the substrate changed as a result of material depositionin the irradiated region, and/or whether the reflectivity indicates thata sufficient thickness of material has been deposited in that region. Ifthe controller 510 determines that the material has been deposited to asufficient thickness, then controller 510 may instruct stage 520 to movesubstrate 210 such that a different region of the substrate isirradiated by laser 530 and by the probe laser. In addition, a laserheterodyne sensor may be provided that includes a continuous wave (CW)laser configured to generate light that is reflected off of the surfaceof substrate 210 onto a photodetector (not shown). The laser heterodynesensor may monitor minute surface deflections (e.g., deflectionsgenerated by the acoustic wave) to check the acoustic wave profile andamplitude at various location on the substrate.

EXAMPLES

Some non-limiting examples of enhanced molecular mobility on a substrateat reduced bulk temperature will now be described with reference toFIGS. 6A-9B.

Specifically, a series of experiments using gold clusters having lownumbers of gold atoms were used as the atomic or molecular species, anda polished glass ceramic surface, specifically lithium aluminosilicate,was used as the substrate. The experiments were performed at roomtemperature, and unless otherwise noted the images are opticalmicroscope images at 100× magnification. Gold clusters were selected asthe molecular species because their locations on the substrate couldreadily be evaluated using optical emission microscopy, as describedbelow. It is believed that the results described below may begeneralized to other atomic or molecular species and other substrates.

The gold clusters were formed by drop-casting a solution of 8 nm goldnanoparticles on the lithium aluminosilicate substrate. Thenanoparticles then were irradiated with 532 nm light from a Quantaraylaser having an 8 ns pulse with, a repetition rate of 100 Hz, andapproximately <1 mJ per laser pulse. The gold nanoparticles absorbed andwere ablated by the light, generating a plurality of gold clusters ofvarying sizes, that is, containing different numbers of gold atoms thanone another. FIG. 6A is an image of the gold nanoparticles prior toablation, and FIG. 6B is an image of the resulting field of goldclusters following ablation, filtered through a 455 nm optical filter(described in greater detail below), in which a gold cluster ofparticular interest is circled. FIG. 6C is an out-of-focus, filteredimage of the same field, so as to demonstrate that the cluster ofinterest is in the focal plane of the image (rather than a speck of duston the microscope), and FIG. 6D is an in-focus, filtered image of thesame field following lateral translation of the substrate, so as todemonstrate that the gold cluster of interest is coupled to thesubstrate.

As is known in the art and as illustrated in FIG. 7A, different goldclusters have different excitation (dashed lines) and emission (solidlines) characteristics than one another. So as to observe the behaviorof only a single size of gold clusters, specifically clusters havingeight gold atoms (Au₈), the field of gold clusters was irradiated usingbroad-band ultraviolet light (approximately 300-400 nm), so as to excitethe Au₈ clusters. FIG. 7B is an image of the field of gold clustersobtained using a 455 nm filter, which is in the emission band for Au₈clusters, and in which several clusters appear bright. FIG. 7C is animage of the field of gold clusters obtained using a 510 nm filter,which is off the emission band peak for Au₈ clusters, in which differentclusters appear bright. Such results suggest that the bright clusters inFIG. 7B (as well as in FIGS. 6B and 6D) have spectroscopic attributesand likely correspond to Au₈ clusters. An analogous experiment wasperformed but with exciting the clusters with blue light (approximately400-480 nm) instead of ultraviolet light, in which it was observed thatsubstantially no clusters appeared bright using the 455 nm filter, whileothers appeared bright using the 510 nm, suggesting that those clusterslikely correspond to Au₁₃.

The motion of Au₈ clusters responsive to interactions with acousticwaves generated in the lithium aluminosilicate substrate then wascharacterized. A Quantaray Nd-Yag laser operating at 355 nm, <70mJ/pulse was used to generate the acoustic waves. Specifically, FIG. 8Ais an average of approximately 100 “before” images of a specific Au₈cluster that were obtained using a 455 nm filter such as describedabove. The average pixel position of the “before” cluster was determinedto have the coordinates (524,565), as schematically illustrated in FIG.8C. The substrate then was irradiated for 10 minutes with a sequence ofpulses from the above-described Quantaray laser, which were shaped aslines with a pseudo-Gaussian profile, a width of approximately 50microns at FWHM, and a length of approximately 5 mm, and that irradiatedthe substrate at a distance approximately 2.5 to 3 cm away from the Au₈cluster. FIG. 8B is an average of approximately 100 “after” images ofthe same Au₈ cluster as in FIG. 8B, following such irradiation. Theimages were analyzed using optical image processing software to removenascent vibration of the apparatus. It may be seen that the clustermoved perceptibly. Indeed, the average position of the “after” clusterwas determined to have the coordinates (520,580), as schematicallyillustrated in FIG. 8C. Such a change in position corresponds to a netmotion of approximately 4 pixels to the left and 15 pixels down. It wasdetermined that each pixel corresponds to approximately 287.7 nm basedon the measured microscope point spread function, so the cluster's nettravel of 15.5 pixels corresponds to approximately 1.1 microns of motionduring the ten minutes of irradiation, for a rate of approximately 0.11micron per minute or 0.02 nm per laser pulse. It is believed that higherlaser repetition rates may result in higher rates of motion.

FIGS. 8D-8E schematically represent cluster motion that was measuredduring experiments analogous to those described above with reference toFIGS. 8A-8C. Specifically, FIG. 8D schematically represents the resultsof an experiment in which pulses from the Quantaray laser having anaverage power of 251 mW were used to irradiate the substrate in a manneranalogous to that described above. Before the irradiation, an Au₈cluster of interest was observed to have an average pixel coordinate of(136.7,165.9), and after 35 minutes of irradiation to have an averagepixel coordinate of (121.2,167.0). Such a change in position correspondsto a net motion of approximately 15.6 pixels to the left and 1.1 pixelsdown, or a total motion of about 15.64 pixels, or 4.49 microns, with arate of about 0.13 microns per minute, or about 0.02 nm per laser pulse.FIG. 8E schematically represents the results of an experiment in whichpulses from the Quantaray laser having an average power approximately55% higher than those in FIG. 8D were used to irradiate the substrate ina manner analogous to that described above. Before the irradiation, anAu₈ cluster of interest was observed to have an average pixel coordinateof (163.3,164.9), and after 30 minutes of irradiation to have an averagepixel coordinate of (151.1,152.8). Such a change in position correspondsto a net motion of approximately 12.2 pixels to the left and 12.1 pixelsdown, or a total motion of about 17.18 pixels, or 4.94 microns, with arate of about 0.165 microns per minute, or about 0.03 nm per laserpulse. Thus, it may be seen that increasing laser pulse intensity mayprovide molecular species with even greater enhancements in mobility.

FIGS. 9A-9B schematically represent cluster motion that was measuredduring experiments analogous to those described above with reference toFIGS. 8A-8E. Specifically, FIGS. 9A-9B schematically represent theresults of an experiment in which pulses from the Quantaray laser wereused to irradiate the substrate in a manner analogous to that describedabove. Before the irradiation, an Au₈ cluster of interest was defined tohave an average pixel coordinate of (0,0) in FIG. 9A, or an observedpixel coordinate of (443,555) in FIG. 9B, and after 30 minutes ofirradiation to have an average pixel coordinate of (23,15) in FIG. 9A,or average pixel coordinate of (420,540) in FIG. 9B. Such a change inposition corresponds to a net motion of approximately 23 pixels to theleft and 15 pixels down, or a total motion of about 27.5 pixels, or 1.9microns, with a rate of about 63.3 nm per minute, or about 0.01 nm perlaser pulse. After the 30 minutes of irradiation, the laser pulses wereblocked, and 20 minutes later the average position of cluster was againobserved. The cluster then had an average pixel coordinate of (20,14) inFIG. 9A, or of (423,539) in FIG. 9B. Accordingly, it may be inferredthat in the presence of laser-generated acoustic waves, molecularspecies may have enhanced mobility.

The above experiments demonstrate that molecular species may be moved ina desired lateral direction on a substrate using broadband acousticwaves, e.g., by approximately a bond length (0.2-0.3 nm) for every tenapplied laser pulses. To appreciate such a result, consider that thediffusion coefficient of a small molecule in water is on the order of10⁻⁵ cm²/second. If the above data for Au₈ were to be fit to a meansquare displacement law from which a diffusion coefficient could bederived, the resulting value would be approximately 3.7×10⁻⁵cm²/second—comparable to that of a small molecule in water, but on a drysurface and at room temperature. Additionally, it should be noted thatheat-driven diffusion is a “random walk” phenomenon, while the aboveexperiments demonstrate that directed motion may be achieved usingacoustic waves that have been shaped to generate a propagating line wave(which may be referred to as a plane wave in the art). Additionally,because the motion is directed, it also may be used to pattern asubstrate surface with selected molecular species without the use ofmasks or optical lithography. As such, embodiments of the invention notonly facilitate the growth of materials at reduced temperature, but alsomay be used to remove or “clean” unwanted molecular species fromdelicate surfaces, or from catalytic surfaces, or to inhibit suchspecies from ever binding to the surface. For example, catalysts tend toget poisoned or lose efficiency over time, and acoustic waves may beused to rejuvenate such catalysts by inhibiting binding between thecatalyst and undesired molecular species. Or, for example, acousticwaves may be used to inhibit growth or binding of bacteria on surfaces,such as surgical instruments, thus sterilizing the instruments andreducing or obviating the need for chemical or thermal sterilization.

Additionally, because acoustic waves propagate by inducing local strain,and small strain rates (e.g., on the order of 10⁻³/second) have beenobserved to induce nucleation (e.g., annealing, removing) of surfacedislocations, it is believed that certain embodiments of the presentinvention may be used to anneal defects such as kinks, terraces, or slipfaults at the surface of a substrate or a material disposed on asubstrate. As devices become small, the ratio of the surface area tovolume increases. Consequently, surface defects may limit reliability,which may be ameliorated using acoustic waves such as described herein.On the other hand, some types of surface waves may propagate to formsolitons, which do not disperse with wavelength, while other types ofsurface waves may form shock fronts. The soliton waves may carry thesurface energy farther, while the nonlinear or shock front waves may beused to crack or induce defects.

While various illustrative embodiments of the invention are describedabove, it will be apparent to one skilled in the art that variouschanges and modifications may be made therein without departing from theinvention. The appended claims are intended to cover all such changesand modifications that fall within the true spirit and scope of theinvention.

What is claimed:
 1. A method for enhancing mobility of an atomic ormolecular species on a substrate, the method comprising: exposing afirst region of a surface of a substrate to an atomic or molecularspecies that forms a molecular bond with the substrate in the firstregion; directing a laser pulse to a second region of the surface of thesubstrate so as to generate a broadband surface acoustic wave in thesecond region, the broadband surface acoustic wave having spatial andtemporal characteristics selected to alter the molecular bond, thebroadband surface acoustic wave having a bandwidth of at least about 100MHz at full width at half maximum (FWHM); and transmitting the broadbandsurface acoustic wave from the second region to the first region, thebroadband surface acoustic wave altering the molecular bond between thesubstrate and the atomic or molecular species and enhancing lateraltranslation of the atomic or molecular species along the surface of thesubstrate wherein responsive to the alteration of the molecular bond,the atomic or molecular species translates laterally across thesubstrate in a direction defined by the spatial and temporalcharacteristics of the broadband surface acoustic wave.
 2. The method ofclaim 1, wherein responsive to the alteration of the molecular bond, theatomic or molecular species forms a material.
 3. The method of claim 2,wherein the substrate has a damage threshold temperature and wherein inthe absence of the broadband surface acoustic wave, the atomic ormolecular species forms the material only at a reaction temperature thatis higher than the damage threshold temperature of the substrate.
 4. Themethod of claim 3, wherein the substrate comprises an integratedcircuit, a chalcogenide glass, a ZBLAN glass, or a polymer.
 5. Themethod of claim 4, wherein the polymer comprises polycarbonate,poly(methyl methacrylate), polystyrene, polyvinyl chloride, orpolyethylene terephthalate.
 6. The method of claim 4, wherein thematerial comprises silicon nitride, graphene, carbon nanotubes, diamond,titanium dioxide, titanium boride, zirconium oxide, yttria-stabilizedzirconium, boron carbide, boron nitride, or metal.
 7. The method ofclaim 1, wherein the laser pulse has a temporal duration of less thanabout 1 picosecond at FWHM.
 8. The method of claim 1, wherein the laserpulse has a temporal duration of less than about 100 femtoseconds atFWHM.
 9. The method of claim 1, wherein the laser pulse is focused to apoint in the second region of the substrate and the broadband surfaceacoustic wave is transmitted radially from the point.
 10. The method ofclaim 1, wherein the laser pulse is focused to a line in the secondregion of the substrate and the broadband surface acoustic wave istransmitted linearly from the line.
 11. The method of claim 1, whereinthe laser pulse is patterned in the second region of the substrate andwherein the broadband surface acoustic wave has a complex profile. 12.The method of claim 1, wherein the molecular bond comprises a covalentbond, an ionic bond, or a bond based on dipole-dipole interactions,London dispersion force, or hydrogen bonding.
 13. The method of claim 1,wherein the laser pulse has a temporal duration of less than about 10nanoseconds at FWHM.
 14. The method of claim 1, wherein the acousticwave is patterned in the second region of the substrate using a sequenceof pulses from one or more lasers.